The West Sea of the Korean peninsula is a high current and turbid environment that presents very harsh conditions to underwater vehicles and divers. Conventional propeller-driven underwater robots have never conducted precision underwater work in the area due to the strong disturbances from the tidal current. To overcome this problem, the Korea Institute of Ocean Science and Technology (KIOST) designed a six-legged underwater robot named Crabster CR200, based on the characteristics of crabs and lobsters.

The underwater hexapod robot CR200 was developed and tested in June and July 2013, with funding from the Ministry of Oceans and Fisheries. CR200 is intended for inspecting shipwrecks and scouring and surveying seafloor tomography in high current and turbid water down to 200 meters, where currents can reach speeds of 3 knots and visibility is less than 1 meter. The underwater robot deals with the high current by crawling on six articulated legs and deals with high turbidity using acoustic equipment. In high current, CR200 aligns its heading to the direction of the current and keeps its nose down and tail up. Therefore, the downward lift force is increased and the robot secures enough contact forces between its feet and the seafloor to guarantee tumble stability. The shape of the body and legs are streamlined to enhance the hydrodynamic characteristics in a high-current environment. The stable posture enables CR200 to get clear acoustic images from the high-resolution scanning sonar even in a highly turbid environment.

Configuration of Equipment
CR200 measures 2.4 by 2.4 by 1.3 meters, weighs 600 kilograms, has a maximum depth of 200 meters, a maximum power of 20 kilowatts and a maximum walking speed of 0.5 meters per second. CR200's six legs are installed along both sides of its body. Control and electric systems are contained in two pressure housings. The two oil-filled junction boxes installed on both sides of the body distribute the power and signal from the pressure housing to the devices. The extendibility of CR200 is achieved by the large junction boxes that support various equipment such as legs/arms, sensors, cameras, lamps, drawer actuator and pan/tilt device. The acoustic Doppler current profiler (ADCP), Teledyne RD Instruments (Poway, California) WHM600, is installed on the upside of the body to detect the speed and direction of the current. CR200 estimates the external forces based on the ADCP data and contact forces of the feet. The high-resolution scanning sonar, Kongsberg Mesotech Ltd. (Port Coquitlam, Canada) MS 1000/1171, and the ultrashort baseline (USBL) responder, Applied Acoustic Engineering Ltd. (Great Yarmouth, England) Easytrak Nexus EZT-2690, are installed on the top of the body. The scanning sonar provides high-resolution acoustic images around the robot. The USBL responder provides return pings to the surface system so that the underwater position of the robot can be traced. The color HD zoom camera, AXIS Communications (Lund, Sweden) AXIS Q6035, and acoustic camera, Sound Metrics Corp. (Bellevue, Washington) ARIS Explorer 3000, on the pan/tilt device provide the optic and acoustic images in real time. The six optical cameras, DeepSea Power & Light (San Diego, California) Nano SeaCam, Wide-i SeaCam and SeeSnake SS-30C, are attached on each side of the body, with three on each side, for visual inspection around the body.

The links and joints of the legs are made of aluminum, and all of the electric motors and harmonic drives are contained in the watertight aluminum housing. The legs and body are covered by fiber-reinforced plastic skins. The body frame of CR200 is made of carbon fiber-reinforced plastic (CFRP) so that it is lighter and stronger than conventional underwater vehicles. Stiffness and strength of the CFRP are 1.5 and 4.4 times higher than Aluminum 6061-T6. Adopting CFRP as a body frame reduces the load on the legs and achieves improvement in strength.

The front two legs of CR200 act as combination arms and legs, and the other four legs are dedicated legs. Each arm-leg combo has seven joints, while the dedicated legs have four joints. Each arm-leg uses four joints in hexapod walking and seven joints in manipulation. Three joints for each leg are usually sufficient for the hexapod gait, but using four-joint legs, which have one redundant joint, optimizes the robot posture for the current or uneven seafloor with redundancy. The length ratio of thigh to shank was determined as 3:4 from biological data of creatures such as lobsters, crabs and insects. Frameless hollow motors and harmonic gears satisfy the requirements of downsizing, lightweight and zero backlash. The absolute encoder was attached to each joint to initialize the joint angle from the incremental encoder with absolute joint angle. All parts of the joint are contained in the aluminum casing for pressure resistance and watertightness. The fabricated leg and arm were tested in a pressure chamber up to 25 bar.

Remote Control System
Once CR200 is launched, all control and monitoring are conducted in the control room. A 20-foot standard shipping container based at KIOST contains all of the equipment for controlling CR200, such as the power supply, communication system, and control and monitoring system. There are seven computers and nine LCD monitors. The agent computer, a gateway that functions as a link between the equipment on CR200 (legs, sensors and actuators) and the computers in the remote control room, is a real-time controller based on the Linux operating system. The pilot and copilot computers provide the graphic user interface and joystick interface to the operators. The video computer displays and saves all of the video data from CR200 and its onboard cameras. The navigation computer displays the locations of CR200, mother ship and target points using the GPS and USBL data. The navigation computer provides planning functions for survey and inspection of the target area. The sonar computer displays real-time acoustic images from the acoustic camera and scanning sonar. The mission computer was installed to support special mission sensors that may be installed in the future, such as a magnetic field sensor, sub-bottom profiler and laser scanner. To continue this article please click here.

Bong-Huan Jun received his B.S. and M.S. degrees in mechanical engineering from the Pukyong National University, Busan, South Korea, in 1994 and 1996, respectively, and his Ph.D. in the Department of Mechatronics Engineering, Chungnam National University, Daejeon, South Korea, in 2006. He is currently a principal research scientist at the Korea Institute of Ocean Science and Technology.

Hyungwon Shim received his B.S., M.S. and Ph.D. degrees in the Department of Mechatronics Engineering, Chungnam National University, Daejeon, South Korea, in 2003, 2005 and 2010, respectively. He is currently a senior research scientist at the Korea Institute of Ocean Science and Technology.

Banghyun Kim received his B.S., M.S. and Ph.D. degrees in computer science from Yonsei University, South Korea. He is currently a research fellow at the Korea Institute of Ocean Science and Technology. His research interests are in computer systems and software for ROVs and AUVs.

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